Composition for controlling scale formation in aqueous systems

ABSTRACT

A method and composition for controlling the formation and deposition of scale forming salts in a circulating aqueous system is described wherein a treatment solution of a substoichiometric amount of polyepoxysuccinic acid in combination with agaric acid is employed.

This is a divisional of application Ser. No. 08/099,674 filed Jul. 29,1993 now U.S. Pat. No. 5,326,478.

FIELD OF THE INVENTION

The present invention relates to the treatment of water to inhibit theformation of scale. More particularly, the present invention relates toa treatment for circulating aqueous systems which comprises apolyepoxysuccinic acid in combination with agaric acid.

BACKGROUND OF THE INVENTION

Although the present invention has general applicability to any givensystem where the formation and deposition of scale and in particularcalcium scale is a potential problem, the invention will be discussed indetail as it concerns circulating cooling water systems. The presentinvention relates to methods for inhibiting scale deposits and foulingin circulating aqueous systems.

In industrial cooling systems, water such as from rivers, lakes, ponds,etc., is employed as the cooling media for heat exchangers. Such naturalwaters contain large amounts of suspended material such as silt, clayand organic wastes. The cooling water circulating through a heatexchanger is typically circulated through a cooling tower, spray pond orevaporative system prior to discharge or reuse. In such systems, coolingis achieved by evaporating a portion of the water passing through thesystem. Because of the evaporation which takes place during cooling,suspended materials in the water become concentrated. Fouling materialsfrom the feedwater or as a result of the evaporative concentration cansettle in locations of low flow rates and cause corrosion andinefficient heat transfer. Agglomerating agents such as polyacrylamidesand polyacrylates have been used to agglomerate fine particles of mudand silt into a loose floc for removal. However, these flocs tend tosettle in cooling tower basins and frequent cleaning is necessary toremove the settled flocs from the tower basins.

The water employed in industrial cooling water systems also oftencontains dissolved salts of calcium, magnesium, etc., which can lead toscale and sludge deposits. One of the most common scale deposits incirculating aqueous cooling systems is calcium carbonate. It normallyresults from the breakdown of calcium bicarbonate, a naturally occurringsoluble salt. Calcium carbonate has a relatively low solubility and itssolubility decreases with increasing temperature and pH. Thus, the rateof calcium carbonate deposition increases with increasing pH andtemperature.

Deposit control agents such as phosphates, phosphonates andpolyacrylates are often used to inhibit calcium carbonate scaleformation in industrial cooling water systems. The use of polyacrylatesalone is not effective at high calcium concentrations becauseundesirable polyacrylate-calcium adducts are formed reducing efficacy.Although phosphonates are very efficient at controlling calciumcarbonate scale formation, they can produce insoluble F,hosphonate-calcium complexes or calcium phosphate scale upondegradation. Further, current limits on phosphate discharge limit theacceptability of the use of phosphonates for water treatments.

Preventing the corrosion and scaling of industrial heat transferequipment is essential to the efficient and economical operation of acooling water system. Excessive corrosion of metallic surfaces can causethe premature failure of process equipment requiring down time for thereplacement or repair of the equipment. Additionally, the buildup ofcorrosion products on heat transfer surfaces impedes water flow andreduces heat transfer efficiency thereby limiting production orrequiring down time for cleaning. Reductions in efficiency will alsoresult from scaling deposits which retard heat transfer and hinder waterflow.

Scale can also cause rapid localized corrosion and subsequentpenetration of metallic surfaces through the formation of a differentialoxygen concentration cells. The localized corrosion resulting fromdifferential oxygen cells originating from deposits is commonly referredto as "under deposit corrosion". The treatment of industrial waters toinhibit scale formation with polyepoxysuccinic acid (hereinafter PESA)is disclosed in U.S. Pat. No. 5,062,962 incorporated herein byreference. The general formula for PESA is: ##STR1## wherein n rangesfrom about 2 to 50, preferably, 2 to 25, M is hydrogen or a watersoluble cation such as Na⁺, NH₄ ⁺ or K⁺ and R is hydrogen, C1-C4 alkylor C1-C4 substituted alkyl (preferably R is hydrogen). PESA is known tobe an effective inhibitor for scale control. However, it was found thatwhen the PESA was employed in some systems that contained a variety ofmaterials, both metal and plastic, and where the water was circulating,there was a slight decrease in efficacy of the scale inhibiting anddeposit control properties of PESA.

SUMMARY OF THE INVENTION

The present invention provides an effective composition and method forinhibiting scale formation and deposition in circulating aqueous systemsby employing PESA in combination with agaric acid. The combination ofthe present invention provides for enhanced scale inhibiting and depositcontrol properties of PESA in circulating water systems. The combinationof the present invention provides for scale formation inhibition anddeposit control in circulating aqueous systems which is effective atconditions of high pH, high calcium concentration and high M-alkalinitywhere conventional calcium control treatments lose efficacy. The presentinvention controls calcium scale formation and the fouling of heattransfer surfaces but does not employ phosphorus thereby eliminating theundesirable discharge of phosphorus containing compounds. The presentinvention is effective at treating circulating waters having low levelsof calcium as well as those having high calcium levels.

The present invention is effective at inhibiting the formation anddeposition of scale forming materials such as calcium oxalate, calciumsulfate, barium sulfate as well as the more common calcium carbonate.The present invention is effective at controlling scale deposition onmetal surfaces such as Admiralty brass as well as plastics such aspolyvinyl chloride and poly(methylmethacrylate) type polymers. Thepresent invention is effective at inhibiting calcium carbonate at highpH as would be required in paper mills.

The method of the present invention comprises treating circulatingindustrial water with a combination of a polyepoxysuccinic acid of thegeneral formula: ##STR2## wherein n ranges from about 2 to 50,preferably 2 to 25, and M is hydrogen or in water soluble cation such asNa⁺, NH₄ ⁺ or K⁺ and R is hydrogen, C1-C4 alkyl or C1-C4 substitutedalkyl (preferably R as hydrogen); and Agaric Acid.

In the present invention, the combination is added to circulatingaqueous systems at substoichiometric levels to inhibit scale formationand deposition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel composition and method ofinhibiting the formation and deposition of scale such as calcium scalefrom circulating aqueous systems. Specifically, the present inventioncomprises adding to a circulating aqueous system a treatment solutioncomprising a combination of a polyepoxysuccinic acid of the generalformula: ##STR3## wherein n ranges from about 2 to 50, preferably 2 to25, M is hydrogen or a water soluble cation such as Na⁺, NH₄ ⁺ or K⁺ andR is hydrogen, C1-C4 alkyl or C1-C4 substitute alkyl (preferably R ishydrogen); and agaric acid or compounds having similar structure andfunctionalities.

Polyepoxysuccinic acids are known to provide calcium scale inhibitioncomparable to prior art phosphates, phosphonates and polyacrylateswithout the recognized limitations of the prior art treatments.Polyepoxysuccinic acids are effective in most water systems, andparticularly effective in aqueous systems having high LangeliefSaturation Index (LSI) numbers. U.S. Pat. No. 5,062,962 (incorporatedherein by reference) outlines a method of preparing a polyepoxysuccinicacid material useful in the present invention.

In the testing of the scale control properties of polyepoxysuccinic acidthe test method measured surface deposition or surface fouling in adynamic or circulating system. The inventors of the present inventiondiscovered that the efficacy of polyepoxysuccinic acid at controllingthe surface deposition in circulating systems was enhanced by theaddition of Agaric acid. Agaric acid, 2-hydroxy-1,2,3, nonadecanetricarboxylic acid, is also called Agaricic Acid.

The treatment levels of polyepoxysuccinic acid added to the circulatingaqueous system can range from 25 parts per billion to about 500 partsper million. The preferred treatment levels range from about 5 parts permillion up to about 100 parts per million. The concentration ofpolyepoxysuccinic acid necessary to provide effective scale control willvary from system to system. The treatment will vary, in part, withchanges in temperature, pH and calcium concentration. However, in allcases, the concentration of polyepoxysuccinic acid added to acirculating aqueous system in accordance with the present invention isat substoichiometric concentrations. That is, the concentration ofpolyepoxysuccinic acid added is much lower than the concentration of thescale forming material in the system to be treated.

The treatment levels of Agaric acid added to the system can range fromabout 20 parts per billion up to about 50 parts per million. Thepreferred treatment level ranges from about 0.1 parts per million up toabout 10 parts per million. The ratio of polyepoxysuccinic acid toagaric acid can range from about 100 to 1 to about 1 to 1.

The present invention will now be described with reference to a numberof specific examples which are to be regarded solely as illustrative andnot as restricting the scope of the present invention.

EXAMPLES EXAMPLE 1

Dynamic beaker calcium carbonate inhibition testing of polyepoxysuccinicacid and polyepoxysuccinic acid/agaric acid blends at varying treatmentlevels were undertaken. The polyepoxysuccinic acid employed in thetesting was prepared as described in U.S. Pat. No. 5,062,962. Thetesting involved adding the treatment to a solution containing calciumand carbonate ions and having a pH of 8.7 at 56° C. Coupons of thematerials to be tested were suspended in the solution. The beakers wereincubated in a shaking water bath for 66 hours at 56° C. After cooling,a measured portion was filtered, and the pH adjusted to less than 2.0with hydrochloric acid. The mixture was diluted and the pH adjusted to12 with sodium hydroxide. A calcium indicator, Murexide, was added andthe solution titrated to a pure violet endpoint with ethylenediaminetetraacetic acid. Percent inhibition was calculated from titrations ofthe treated, stock and control solutions. Deposition on the suspendednon-heat transfer surfaces was evaluated by visual inspection. Theconditions of the tests were: 600 ppm Ca as CaCO₃ ; 300 ppm Mg as CaCO₃; 400 ppm Malk as CaCO₃ ; 288 ppm SO₄ ; 425 ppm Cl; 185 ppm Ma; pH 8.7at 56° C.; 3 ppm tolyltriazole.

Table 1 summarizes the results of the tests and shows that thecombination of the present invention improves the deposit rating for allof the materials tested.

                  TABLE 1                                                         ______________________________________                                                        %        Coupon    Deposition*                                Treatment       Inhibition                                                                             Material  Rating                                     ______________________________________                                        Control         --       Admiralty 4                                          10 ppm PESA     54.0     Admiralty 2                                          20 ppm PESA     83.0     Admiralty 2                                          2 ppm Agaric acid                                                                              3.0     Admiralty 3                                          10 ppm PESA + 2 ppm                                                                           65.5     Admiralty 2                                          agaric acid                                                                   20 ppm PESA + 2 ppm                                                                           80.0     Admiralty 1                                          agaric acid                                                                   20 ppm PESA + 1 ppm                                                                           80.5     Admiralty 1                                          agaric acid                                                                   20 ppm PESA     81.0     Admiralty 2                                          20 ppm PESA + 0.2 ppm                                                                         78.0     Admiralty 1                                          agaric acid                                                                   20 ppm PESA + 0.4 ppm                                                                         78.0     Admiralty 1                                          agaric acid                                                                   20 ppm PESA + 0.8 ppm                                                                         78.0     Admiralty 1                                          agaric acid                                                                   Control         --       Plexiglas ®                                                                         4                                          10 ppm PESA     63.3     Plexiglas ®                                                                         3                                          20 ppm PESA     83.0     Plexiglas ®                                                                         4                                          10 ppm PESA + 2 ppm                                                                           66.5     Plexiglas ®                                                                         2                                          agaric acid                                                                   20 ppm PESA + 2 ppm                                                                           80.0     Plexiglas ®                                                                         1                                          agaric acid                                                                   20 ppm PESA + 1 ppm                                                                           95.0     Plexiglas ®                                                                         1                                          agaric acid                                                                   20 ppm PESA     81.0     PVC       2                                          20 ppm PESA + 0.2 ppm                                                                         78.0     PVC       2                                          agaric acid                                                                   20 ppm PESA + 0.4 ppm                                                                         76.0     PVC       1                                          agaric acid                                                                   20 ppm PESA + 0.8 ppm                                                                         78.0     PVC       1                                          agaric acid                                                                   ______________________________________                                         *1 = clean                                                                    2 = very slight deposit                                                       3 = slight deposit                                                            4 = moderate deposit                                                          Plexiglas ® is a trademark of Rohm & Haas Co.                        

EXAMPLE 2

Testing was conducted in a bench top unit (BTU) which simulates acirculating cooling water system. The BTU units are designed to providea measure of the ability of the treatment to prevent corrosion and scaleformation under heat transfer conditions. The treated water iscirculated through a corrosion coupon by-pass rack, into which corrosioncoupons are inserted and passes through a heat exchanger tube containedin a Plexiglas® block (trademark of Rohm & Haas Company). The heatexchanger is fitted with an electrical heater so that the heat load onthe exchanger tube can be varied and controlled in the 0 to 16,000BTU/ft² /hr range. The water velocity passing the unit can be controlledin the 0 to 4.5 ft/sec. range. A section of PVC tower fill is immersedin the sump of the unit.

The pH and temperature of the circulating water are automaticallycontrolled. The treated water was prepared by the addition of thecomponent ions to deionized water. Provisions for continuous makeup andblowdown are made by pumping fresh treated water from supply tanks tothe sump of the unit, with overflow from the sump serving as blowdown.The system volume is approximately 12 liters. Table 2 summarizes testingin these bench top units for a variety of materials. The test conditionswere: 600 ppm Ca as CaCO₃ ; 300 ppm Mg as CaCO₃ ; 400 ppm Malk as CaCO₃; 288 ppm SO₄ ; 425 ppm Cl; 187 ppm Na; 3 ppm tolyltriazole; pH 8.8;bulk temperature 49° C.; skin temperature 57°; flow rate 2.8feed/second; heat input 13,000 BTU/ft² /hr.

Table 2 summarizes the results of the bench top unit testing and showsthat the combination of the present invention enhances the depositcontrol activity of polyepoxysuccinic acid in circulating systems.

                                      TABLE 2                                     __________________________________________________________________________    Dynamic CaCO.sub.3 Inhibition                                                                    PVC*                                                                      Plexi-                                                                            Twr.                                                                 Tube*                                                                              glas*                                                                             Fill                                                                              Coup*                                                                             Corr.                                                        Depos.                                                                             Block                                                                             Depos.                                                                            Depos.                                                                            Rate                                                                              Dura-                                                                             Tur-                                       Treatment                                                                            ppm                                                                              Rating                                                                             Rating                                                                            Rating                                                                            Rating                                                                            mpy tion                                                                              bidity                                     __________________________________________________________________________    PESA   20 4    4   4   4   0.1 7   5.86                                       PESA   20  2** 1   1   1   0.2 14  2.32                                       Agaric Acid                                                                          2                                                                      PESA   20 2    1   1   No  0.35                                                                              7   0.87                                       Agaric Acid                                                                          2                                                                      PESA   20 1    1   1   1   0.2 7   0.50                                       Agaric Acid                                                                          1                                                                      PESA   20 1    1   1   1   0.35                                                                              4   0.41                                       Agaric Acid                                                                          0.5                                                                    PESA   20   2***                                                                             3   1   1   1.25                                                                              7   0.49                                       Agaric Acid                                                                          0.2                                                                    __________________________________________________________________________     *1 = clean                                                                    2 = very slight deposit                                                       3 = slight deposit                                                            4 = moderate deposit                                                          **deposit consisted of agaric acid                                            ***deposit consisted of calcium carbonate                                

While the present invention has been described with respect toparticular embodiments thereof, it is apparent that numerous other formsand modifications of the invention will be obvious to those skilled inthe art. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true scope and spirit of the present invention.

We claim:
 1. A composition for inhibiting the deposition of scale incirculating aqueous systems comprising an aqueous solution ofpolyepoxysuccinic acid and agaric acid in a ratio of from about 100 to 1to about 1 to
 1. 2. The composition of claim 1 wherein saidpolyepoxysuccinic acid is present in said aqueous solution in levelsranging from about 25 parts per billion to about 100 parts per million.3. The composition of claim 1 wherein said agaric acid is present insaid aqueous solution in levels ranging from about 20 parts per billionup to about 10 parts per million.